Process for treating polyethylene structures



Aug. 9, 1955 L. E. WOLINSKI 2,715,077

PROCESS FOR TREATING POLYETHYLENE STRUCTURES Filed Nov. 29, 1952 INVENTOR LEONE. WOLINSKI BY %%/%;A..

ATTORNEY United PRUCESS FGR TREATING POLYETHYLENE STRUCTURES Application November 29, 1952, Serial No. 323,276

3 Claims. (Cl. 117-47) This invention relates to a process of treating the sur face of polyethylene structures and, more particularly, to a process of treating the surface of a polyethylene film to promote the adhesion thereto of printing inks and various other materials.

U. S. Patent 2,219,700 to Perrin et al. discloses and claims a polyethylene film, i. e., a film of a solid polymer of ethylene. in general, polyethylene films are tough, semi-transparent, resistant to many chemicals, exhibit a high degree of moisture vapor impermeability, permit the passage of oxygen, and are heat-scalable. Because of this combination of properties, polyethylene films are highly useful for packaging and wrapping a great variety of materials such as chemicals, fresh produce, dried milk, textiles, hardware, etc. Probably the only troublesome disadvantage of polyethylene film for use in the packaging field is the fact that standard aniline and rotogravure printing inks employed for printing various cellulosic films, such as re euerated cellulose and cellulose acetate films, do not adhere satisfactorily to the surface of the film. Generally, any indicia, such as trademarks, advertising indicia, recipes, etc., imprinted upon a surface of a polyethylene film with standard oil or lacquer type inks employed for printing cellophane film are easily smeared or rubbed off by the normal abrasions suffered by packages during shipping, handling, etc. Hence, in order to obtain satisfactory adhesion between a dried ink and a polyethylene film surface, it is necessary to employ a specially compounded ink or modify the film surface to promote improved ink adhesion.

Although printing inks compounded particularly for printing on polyethylene films have been developed, the use of most of these inks requires modification of standard printing processes; and the preferred approach is treatment of the polyethylene film surface to promote adhesion of standard oil and lacquer type inks.

An object of the present invention is to provide a process of treating the surface of a polyethylene structure, e. g., film, to improve adhesion of standard printing inks, i. e., promote adhesion of standard aniline and rotogravure inks employed in printing on cellophane film. Another object is to provide a process of treating the surface of a polyethylene film to improve adhesion thereof to various other materials, such as metals, paper, nitrocellulose coatings, and other polymeric coatings, e. g., nylon, polyethylene terephthalate, etc. A further object is to improve the adhesion of polyethylene film to itself and other materials when using commercial adhesives. A still further object is to provide a process of treating the surface of a polyethylene film to improve adhesion thereto of dried ink impressions and not impair the transparency of the film. A still further object is to provide a polyethylene film having modified surface characteristics such that dried ink imprints on the surface will not rub off when tested in accordance with the various tests described hereinafter. Other objects will be apparent from the following description of the invention.

tates atent ffatented An The objects are realized by the present invention which, briefly stated, comprises subjecting a polyethylene structure, e. g., polyethylene film, at a temperature within the range of from about 150 to about 325 C. to the action of nitrous oxide, preferably in the presence of ultra-violet light.

It should be understood that 325 C. is not an upper limit insofar as the operability of the present invention is concerned. Extrusion of presently available polyethylene compositions at temperatures substantially higher than 325 C. is not practical because the melt is too fluid, and presently known antioxidants do not erficiently prevent degradation of the polymer at appreciably higher temperatures. With the development of polyethylene compositions which form a more viscous melt and the discovery of more efficient antioxidants, extrusion may be carried out more rapidly and efiiciently at temperatures of 400 C. and above, the maximum being that temperature beyond which substantial degradation of the polyethylene composition occurs.

In the normal process of extruding molten polyethylene into film form, a molding powder or flake of polyethylene is fed continuously into a melt extrusion machine, and the molten film continuously extruded through a slot orifice and through an air gap vertically downward into a quench bath maintained at a temperature from 25- C., preferably from 30-60 C. Usually, the polyethylene is extruded from a melt maintained at a temperature within the range from to 325 C. Tubing is usually extruded from a melt at a temperature Within the range from l50-200 C., whereas film is extruded at a temperature within the range from 250-325 C. An alternative process of forming a polyethylene film comprises milling molten polymer on closely-spaced calender rolls to form a film which is conducted vertically downward into a quench bath. in either of these general methods of forming a polymeric film, the space between the point where the molten film leaves the slot orifice or the last calender roll and the point where the molten film enters a quench bath will hereinafter be termed the air gap. During passage throught the air gap, the film is merely permitted to pass uninhibited through the atmosphere; and this provides for some superficial cooling. Generally, the length of the air gap ranges from about 2" to as long as 15 in some cases.

Because of the rapid action of nitrous oxide upon the surface of polyethylene film at elevated temperatures, the process of the present invention is most conveniently carried out by subjecting freshly extruded film at a temperature of from about 150 to about 325 C. to the action of nitrous oxide as the film passes through the air gap. For example, freshly extruded film may be treated in accordance with this invention by suitably enclosing the air gap and providing for the maintenance in the enclosure of an atmosphere, e. g., air, containing nitrous oxide. Provision may be made for the employment of ultra-violet light by making the Walls of the enclosure transparent to ultra-violet light or by installing a source of ultra-violet light inside of an opaque enclosure. Hence, the process of the present invention may be carried out by making very simple modifications to existing film-forming or tube-forming apparatus; and, owing to the rapid action of nitrous oxide, employment of the present process does not preclude production of film at commercially satisfactory rates. Normally, in order that polythene film may be treated in accordance with the present invention as part of presently employed extrusion or calendering techniques of forming the film, the time of treatment should be no greater than about 2 seconds in order to permit operation at commercially acceptable rates.

Treatment of the film maintained at an elevated temperature is essential for obtaining rapid modification of the film surface with nitrous oxide. Normally, treatment at temperatures substantially below 150 C. does not provide for obtaining rapid action with nitrous oxide.

For effective action within the relatively short exposure time permissible in the treatment of film being extruded at commercially acceptable rates, the concentration of nitrous oxide in the treating atmosphere must be at least 5%, by volume of the total volume of gases present in the treating atmosphere. Optimum results are generally obtained using nitrous oxide. Nitrous oxide concentrations in excess of 20% and up to 100% do not tend to burn the film, i. e., impair transparency or semitransparency of the film, and may be used where very short exposure times are required.

Preferably, ultra-violet light having a wave length no greater than 3900 AU is employed to accelerate the action of nitrous oxide, particularly where the process of this invention is incorporated in the continuous production of film.

The following examples illustrate the preferred practice of this invention, reference being had to the accompanying drawing wherein is shown diagrammatically an arrangement of apparatus used in carrying out the process of this invention.

Referring to the drawing, molten polyethylene, at a temperature of 265 C., was extruded in the form of a film F from extrusion hopper 1 into the air gap surrounded by an enclosure constituting a treating chamber 2, the walls of which are formed in part, at least, of transparent material (quartz glass) to provide for the transmission therethrough of ultra-violet light emitted from mercury arc lamps 3 placed two inches from the film. The upper end of the chamber was closed by the extrusion hopper, and the bottom of the chamber was sealed from the atmosphere by projecting the sides thereof below the surface of the cooling water C.) in the quench bath 4. The length of the air gap was 10 inches, and the path of travel of the film in the quench bath was also 10 inches. Air at atmospheric pressure and containing 20, by volume, of nitrous oxide was passed into the chamber at 5 and out at 6. At no time did the temperature of film in chamber 2 drop below 200 C. The treatment times set forth in the following table are the actual times that any given increment of the film remained in the treating chamber.

In evaluating the printability, i. e., the strength of the adhesive bond between the dried ink and the treated polyethylene film surface, a number of tests were employed (5 in all); and on the basis of the results of all of the tests, the films were rated either acceptable or not acceptable, and, if acceptable, either excellent or good. Four different inks were employed to print the treated surfaces of polyethylene film, and each printed sample was evaluated in accordance with each of the five tests which will be described hereinafter. The inks employed were as follows:

No. 1.Aniline cellophane ink (Bensing Bros. and Deeney, No. W400).

No. 2.Aniline polyethylene ink (Interchemical Corporation, No. PARed).

No. 3.-Rotogravure cellophane ink (Bensing Bros. and Deeney, No. G1037).

No. 4.-Rotogravure polyethylene ink (Interchemical Corporation, IN-Tag-Red, GPA Red).

In preparing the printed samples of polyethylene film, the ink was applied with a commercial ink spreader which comprised a steel rod having fine wire wrapped around the rod. The spreader produced a multiplicity of fine lines. The ink was then dried for three minutes at C. and thereafter permitted to cool to room temperature. Each sample was then tested in accordance with each of the following tests, and the amount of ink rubbed off and/or removed was noted:

1. Rub Test.-The inked polyethylene surface was rubbed ten times against a hard white paper.

2. Scratch T est.The back of a fingernail was rubbed across the inked surface.

3. Flex Test.-The film Was held between thumb and forefinger (2 apart) and flexed vigorously.

4. Pressure-Sensitive Ta e T est-A pressure-sensitive tape was pressed against the printed surface, and then pulled off.

5. Twist T est-The printed film was folded once and then again in a direction perpendicular to the first fold. The folded. ends were then twisted once around and thereafter the film surface was examined for smearing and/or cracking of the dried ink.

Although it is convenient and preferable to treat the polyethylene film with nitreous oxide in the air gap between the extrusion orifice and the quench bath as described above, or between the last calender roll and the quench bath, the film being at a temperature close to the actual melt extrusion temperature, the process of the present invention may be applied to the film at a point after the film has been quenched. To carry out the process of the present invention at this point would require conducting the film through a hot air channel or oven for the purpose of reheating the film to a suitable temperature, i. e., above C., and immediately thereafter or simultaneously treating the film in a confined atmosphere containing nitrous oxide.

Moreover, while the present process is employed primarily for treating the surface of a polyethylene film in order to produce a film which may be successfully printed with standard oil or lacquer type inks, e. g., aniline or rotogravure inks employed for printing on cellophane film, the present invention may be employed to modify the surface of a polyethylene film which is to be printed with inks which are especially modified for printing upon a polyethylene film surface. The net result is an even further improvement in the adhesive bonds between the dried ink and the polyethylene film surface. The present invention further provides for the preparation of a polyethylene film which is more readily adherent to metals, papers, and various coatings, such as those of nitrocellulose; polyamides, e. g., polyhexamethylene adipamide, polyhexamethylene sebacamide, N-methoxymethyl polyhexamethylene adipamide and other polyamides defined in U. S. P. 2,430,860, and interpolyamides defined in U. S. P. 2,285,009; polyethylene terephthalate; polyvinyl acetals such as polyvinyl butyral; ethyl cellulose; vinyl acetate-vinyl chloride copolymers; vinylidene chloride copolymers; chlorinated rubbers; etc. Furthermore, polyethylene film treated by the present process is more readily adhered to itself and other base materials by using commercial adhesives, e. g., standard adhesives employed for sealing cellophane.

The process of this invention may also be employed for treating the surface of various films fabricated from copolymers of ethylene with various other polymerizable materials, e. g., isobutylene, vinyl acetate, styrene, vinyl chloride.

The outstanding advantage of the present process is that it provides a readily applicable and rapid method of improving the adhesion of a dried printing ink to the surface of a polyethylene film. The process may be readily combined with a necessary step of extruding or calendering molten polyethylene into film or tube form, and the additional apparatus required is inexpensive and easy to install.

Another outstanding advantage of the present invention is that it provides for the preparation of an improved polyethylene film which forms heat seals of higher bond strength than seals made with films treated by any other known process of improving the adhesion of printing inks to polyethylene film. This is especially true of film which has been treated with a sizing composition, such as an aqueous solution of an alkyl aryl polyglycol ether of the type defined in U. S. P. 2,519,013.

As many widely different embodiments may be made without departing from the spirit and scope of this in vention, it is to be understood that said invention is in no wise restricted except as set forth in the appended claims.

I claim:

1. A process for treating structures of polyethylene which comprises subjecting said structures to the action of j a gaseous atmosphere containing at least by volume of nitrous oxide at a temperature within the range of from 150 C. to the temperature beyond which substantial degradation of the polyethylene occurs, for a period of time suificient to render said structures adherent to printing ink.

2. A process according to claim 1 wherein the temperature is within the range of from about 150 to about 325 C.

3. A process for treating structures of polyethylene which comprises subjecting said structures to the action of a gaseous atmosphere containing at least 5% by volume of nitrous oxide at a temperature within the range of from 150 C. to the temperature beyond which substantial degradation of the polyethylene occurs, in the presence of ultra-violet light having a wave length no greater than 3900 A. U., for a period of time sufficient to render said structures adherent to printing ink.

4. A process for treating polyethylene film which comprises subjecting said film to the action of a gaseous atmosphere containing at least 5% by volume of nitrous oxide at a temperature within the range of from C. to the temperature beyond which substantial degradation of the polyethylene occurs, for a period of time sulficient to render said film adherent to printing ink.

5. A process according to claim 4 wherein the temperature is within the range of from about 150 to about 325 C.

6. A process for treating polyethylene film which comprises subjecting said film to the action of a gaseous atmosphere containing at least 5% by volume of nitrous oxide at a temperature within the range of from 150 C. to the temperature beyond which substantial degradation of the polyethylene occurs, in the presence of ultraviolet light having a Wave length no greater than 3900 A. U., for a period of time suificient to render said film adherent to printing ink.

7. A process for treating polyethylene film which comprises subjecting said film to the action of a gaseous atmosphere containing at least 5% by volume of nitrous oxide at a temperature within the range of from 150 C. to the temperature beyond which substantial degradation of the polyethylene occurs, for a period of time sufficient to render said film adherent to printing ink, and thereafter imprinting such surface with a printing ink.

8. A process according to claim 4 wherein said atmosphere contains about 20% by volume of nitrous oxide.

References Cited in the file of this patent UNITED STATES PATENTS 2,400,720 Staudinger May 21, 1946 2,499,421 Samler Mar. 7, 1950 2,502,841 Henderson Apr. 4, 1950 2,612,480 May Sept. 30, 1952 2,622,056 De Coudrcs Dec. 16, 1952 2,639,998 Pavlic May 26, 1953 

7. A PROCESS FOR TREATING POLYETHYLENE FILM WHICH COMPRISES SUBJECTING SAID FILM TO THE ACTION OF A GASEOUS ATMOSPHERE CONTAINING AT LEAST 5% BY VOLUME OF NITROUS OXIDE AT A TEMPERATURE WITHIN THE RANGE OF FROM 150* C. TO THE TEMPERATURE BEYOND WHICH SUBSTANTIAL DEGRADATION OF THE POLYETHYLENE OCCURS, FOR A PERIOD OF TIME SUFFICIENT TO RENDER SAID FILM ADHERENT TO PRINTING INK, AND THEREAFTER IMPRINTING SUCH SURFACE WITH A PRINTING INK. 